Lifting force responsive load control for electrically propelled earthmoving vehicles

ABSTRACT

An electrical propulsion control system for self-propelled earthmoving traction vehicles of the type having integral earthmoving means and having a prime mover-driven electric generator supplying energy to traction motors, wherein the electrical output of the generator is regulated within predetermined maximum limits which are reduced in response to the application of a predetermined lifting force to the earthmoving means.

BACKGROUND OF THE INVENTION

This invention relates generally to electric propulsion systems forself-propelled traction vehicles, and it relates more particularly tosuch a system for driving an earthmoving machine wherein a liftingmechanism and the electrical propulsion system are both powered by thesame prime mover.

In one embodiment of the invention to be disclosed hereinafter, theelectric propulsion system is intended to drive earthmoving machinesknown technically as "wheel loaders" (standard J1057 of the Society ofAutomotive Engineers) and popularly as "front end loaders." A front endloader comprises a self-propelled vehicle with an integral front-mountedbucket supporting structure and linkage that loads earth and othermaterials into the bucket through forward motion of the vehicle and thenlifts, transports, and discharges the load. Such a machine typicallyincludes an articulated frame and a four-wheel drive. Both front andrear axles can be driven by an electrical system comprising a pair ofvariable speed reversible d-c motors (each having an armature and afield) which are energized by a generator coupled to a diesel engine orother suitable prime mover, and the bucket and its boom can be poweredby hydraulic means including lift cylinders which derive their hydraulicpressure from the same prime mover. By appropriate manipulation of aspeed-control pedal and a forward-reverse selector lever, an operatorcan control the electric drive system so as to determine, respectively,the machine's speed and direction of movement. The various operatingmodes of such a system include propulsion (motoring) or dynamicretardation (braking) in either a forward or reverse direction, with thebucket either loaded or unloaded; propelling the machine forward withthe bucket down to penetrate a pile of earth ("crowding"); and liftingthe bucket while the wheels are either stationary or moving forward orbackward.

Propulsion systems for front end loaders should preferably have certaincharacteristics including: (1) smooth control of torque, (2) miminalwheel slip for improved tire life, (3) high tractive effort at lowspeeds to permit the loader bucket to readily penetrate the pile, termed"full crowd tractive effort," (4) relatively constant prime mover enginespeed to permit rapid cycling and response of the bucket and boomassembly and to facilitate engine smoke control, (5) controllablevehicle speed, and (6) simplified control, such as, for example, tofacilitate the repeated reversals in direction required duringoperation. Whereas the present invention will be described in connectionwith a propulsion system having the above characteristics andparticularly adapted for front end loaders, it may be utilized in othertypes of electrical drives including those for other types of vehiclesand those providing certain alternative characteristics.

SUMMARY OF THE INVENTION

In the operation of a front end loader, the boom and bucket aremanipulated so as to penetrate and lift earth matter during forwardpropulsion of the vehicle. If the tractive effort of the propulsionsystem and the lifting force of the bucket were too great during thisoperation, the bucket may be undesirably overloaded. Accordingly, ageneral objective of the present invention is the provision, in anelectrical propulsion system for a traction vehicle having anearthmoving bucket or the like, of improved control means forapportioning the traction power and the lifting force.

A more specific objective is to provide an arrangement for reducing themaximum available tractive effort of the electrical propulsion system ofsuch a vehicle during operation of the bucket lifting mechanism, therebygiving preference to the latter.

In carrying out the invention in one form, the traction motors of anelectrically propelled earthmoving traction vehicle such as a front endloader are energized by the output of an electric generator driven by asuitable prime mover which also is adapted to supply power to ahydraulic lifting system for the earthmoving bucket of the loader. Tokeep the electrical output of the generator from exceeding predeterminedsafe limits, suitable means is provided for reducing the generatoroutput when it exceeds predetermined reference levels. The propulsioncontrol system additionally includes means responsive to a lifting forceapplied to the bucket for reducing the aforesaid reference levels,either as a step function of proportionately to the lifting force,thereby reducing the power output limit of the generator (andconsequently reducing the maximum available tractive effort of theproplsion system) during operation of the hydraulic lifting system.

The invention will be better understood and its various objects andadvantages will be more fully appreciated from the following descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an electric propulsion systemincorporating the present invention, which system includes a pair ofmotors energized by a generator driven in turn by a prime mover;

FIG. 2 is a graphic representation of the relationship between outputvoltage and output current of the generator used in the propulsionsystem of FIG. 1;

FIG. 3 is a schematic circuit diagram illustrating a preferredembodiment of the means shown in block form in FIG. 1 for regulating thegenerator output; and

FIG. 4 is a schematic circuit diagram illustrating a preferredembodiment of the means shown in block form in FIG. 1 for controllingmotor field.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 shows a preferred embodiment of an electrical propulsion systemuseful for driving a front end loader or the like. To facilitate anunderstanding of the main parts of this system and of its overalloperation, the following description is organized under separateheadings and preferred means for implementing certain functions in thesystem are shown in greater detail in other figures. After thisdescription, the specification will conclude with claims pointing outthe particular features of the system that are regarded as the subjectmatter of the present invention. Other features of the desired systemare claimed in co-pending patent applications filed concurrentlyherewith and assigned to the assignee of the present invention. Theco-pending patent applications and their titles are as follows:

Ser. No. 553,862

Prime Mover Speed Responsive Load Control For Electrically PropelledTraction Vehicles now U.S. Pat. No. 3,970,858

Ser. No. 553,865

Speed Control For Electrically Propelled Traction Vehicles

Ser. No. 553,861

Field Boost Arrangement For Separately Excited D-C Traction Motors Of AVehicle Propulsion System

Ser. No. 553,866

Field Tapering Arrangement For Separately Excited D-C Traction Motors OfA Vehicle Propulsion System

With particular reference now to FIG. 1, the illustrated propulsionsystem includes a front motor 2 and a rear motor 4 which are intended topropel, or to retard, the front and rear axles, respectively, of thefront end loader by a coupling arrangement schematically indicated bylines 6 and 8. In lieu of a single motor axle, multiple motors may ofcourse be utilized, such as, for example, separate electrically poweredtraction wheels having their armatures connected in parallel or inseries during propulsion, i.e. motoring operation. The electricallypowered traction wheels may be of the general type disclosed in U.S.Pat. No. 2,899,005 Speicher.

Each of the traction motors 2 and 4 is a variable speed, reversible d-cmotor having an armature and a separately excited field. The armaturesof the two motors are connected in parallel for energization by the samevoltage, and as is shown more clearly in FIG. 4, the motor fields areconnected in series with each other for separate excitation by the samefield current. A thermal prime mover 10, such as, for example, a dieselengine, drives, as is indicated by dashed line 14 in FIG. 1, electricalgenerating means 12 which in turn provides in electrical output to theparallel connected armature means of motors 2 and 4, as schematicallyindicated by line 16. A source of field current, field supply means 18,provides field current I_(F), on line 20, to the series connected fieldwindings of motors 2 and 4. In the preferred embodiment, the fieldsupply means 18 is a rotary d-c generator which is also driven by theprime mover 10, as schematically indicated by line 22. The describedarrangement provides substantially identical armature voltages and fieldcurrents, and thus field flux, to the separately excited traction motorsso that each motor maintains identical rotational speeds. This featureassists in minimizing wheel slip.

Motor performance, i.e. motor torque and speed, is a function of themagnitude of applied armature voltage, and thus armature current, andmagnitude of field flux, i.e. applied field current. In the preferredembodiment these parameters are controlled by applying appropriateexcitation to the fields, respectively, of the electrical generatingmeans 12 and of the field supply generator 18. In the illustratedembodiment field excitation for generating means 12 is provided byexciting generator 24, whose output V_(E) is coupled through switch 26to the field circuit of generating means 12. The output of exciter 24constitutes an amplified output of the signal I_(F) applied on line 28to excite the field of the exciter generator. This signal I_(F) isregulated by a regulating system described subsequently.

MOTOR FIELD EXCITATION

The magnitude of motor field flux is determined by the field excitationof field supply generator 19, i.e the magnitude of field currentsupplied on line 30. Preferably the motor field, and thus the fieldcurrent on line 30, is of predetermined constant magnitude under normaloperating conditions, and this magnitude is selected so that motor fluxis above the knee of its saturation curve. The field current 30 may, forexample, be supplied from a constant current source, such as a battery,connected serially with a resistance and a field winding of generator18. As subsequently described, however, improved performance isobtainable by automatically modifying the field current under specialconditions. One occurs when unusual tractive effort is required, such asneeded for the bucket to crowd into a pile. A field current boostcircuit responsive to traction motor currents in excess of apredetermined magnitude automatically provides additional fieldexcitation to increase motor torque. This permits attainment ofadequately high motor torque while maintaining motor armature currentwithin desired limits. The other condition occurs when additionalvehicle speed is required when maximum armature voltage is supplied tothe traction motors. The field current is then automatically reduced toprovide field weakening and extended speed operation. The summationcircuit 32 of FIG. 1 functionally presents an arrangement for thusmodifying field energization. The previously referenced normal statefield current, I_(E), is illustrated as being supplied fromforward-reverse switch 34 and line 36 to summer 32. Under normalconditions this constitutes the sole excitation of the field supplygenerator. Double pole switch 34 functionally illustrates an arrangementfor reversing the rotational direction of the traction motors forreversing the vehicle. Motor reversal is obtained by reversing the fieldexcitation current, such as by reversing the connections between thefield winding and the source of potential supplying the field current.Obtaining reversal by switching the relatively small field currentsupplied to the field supply generator, permits use of switchingdevices, such as contactors, having limited current carrying capability.

In order to obtain the above-referenced field current boost, a currentboost signal, I_(BOOST), is applied from the field current boostcircuit, comprising OR gate 38 and hold off gate 40, by line 42 tosummer 32. Motor armature current signals on lines 56 and 58 aresupplied to OR gate 38 which supplies the current signal of highestamplitude I_(M) on line 39 to hold off gate 40. Gate 40 provides anoutput I_(BOOST) on line 42 in the event signal I_(M) exceeds apredetermined threshold. Preferably I_(BOOST) increases proportionatelywith further increases of signal I_(M).

In order to obtain the above-referenced field weakening, a fieldweakening signal, I_(T), is applied from a field weakening circuit,comprising hold off gate 44, by line 46 to summer 32. Hold off gate 44receives an input V_(G) representative of traction motor voltage oroutput volatage of generating means 12. In the event this signal exeedsa predetermined magnitude, preferably near the maximum rated voltage,gate 44 supplies an output signal I_(M) on line 46.

As indicated in FIG. 1, the boost current on line 42 is combinedadditively, and the field weakening current on line 46 is combinedsubtractively, with the normal field current I_(E) on line 36. In thepreferred embodiment the above-described summation function of summer 32is in fact achieved by utilizing plural field windings on the fieldsupply generator 18.

GENERATOR REGULATING SYSTEM

In order to explain additional features of the system of FIG. 1,reference is now made to the regulating system which controls the outputof the electrical generating means 12. As is known, the voltage outputof means 12 must be maintained within a predetermined magnitude,primarily to protect the field windings of generating means 12 and toprevent dielectric breakdown of the insulating of the entire tractionsystem. Further the current output of means 12 must be maintained withina predetermined magnitude to protect the armature of generating means 12and other circuit components. In addition the power output of thegenerating means 12 must be kept within a predetermined power, e.g."horsepower," limit to prevent overloading the prime mover engine 10 andto prevent stalling of the engine. Essentially the voltage and currentoutput of the generating means 12 are dependent on the load, i.e. theperformance of the traction motors, and are independent of each other.For example, when the vehicle accelerates from standstill the hightorque requirements result in high armature current, i.e. the loadimpedance is very low, and the current must be limited. On the otherhand at high speed and minimal torque operation, the traction motorsdevelop substantial counter voltage, i.e. back emf. This is equivalentto increasing the load impedance. Accordingly the voltage output of thegenerating means increases at high speeds and voltage limiting isrequired. At intermediate levels of operation, the power output of thegenerating means must be limited. The horsepower output limit, being afunction of the products of voltage and current outputs, is hyperbolicin form. The resulting idealized operating envelope is illustrated inFIG. 2, a plot of the output voltage vs. the output current ofgenerating means 12. Line F-G represents the voltage limit, line H-Irepresents the current limit, and hyperbolic curve portion G-Hrepresents the horsepower limit portion of the envelope. The regulatingsystem assumes that the output of the generating means does not exceedthe limits prescribed by the above-described envelope. This is achievedessentially by deriving signals representative of the voltage andcurrent outputs of the generating means, processing these voltage andcurrent signals to provide a signal which is a function of the poweroutput of the generating means, and comparing these signals withappropriate reference signals to derive a control, or error, signal. Thecontrol signal controls the field excitation of the generating means tomaintain output within the desired generator voltage and currentenvelope, i.e. within predetermined maximum voltage, power, and currentparameters. In the preferred embodiment illustrated in FIG. 1, line 16provides voltage feedback signals representative of the actual voltageV_(G) applied from the output of the generating means to the armaturecircuit of the fraction motors, and line 52 provides current feedbacksignals representative of the actual current output supplied by thegenerating means to the armature circuit of the traction motor means. Asillustrated in FIG. 1, this current feedback signal may be derived bydetecting signals representative of the armature currents of the frontmotor means 2, on line 56, and of the rear motor means 4, on line 58,and summing these signals, I_(M1) and I_(M2), in summation device 60 soas to provide the above-referenced current feedback signal, I_(G), online 52. The voltage feedback signal on line 16, and the currentfeedback signal on line 52 are processed by devices 62 and 54 togenerate a signal on line 66 which varies appropriately as a function ofthe power output of the generating means, and thus may be termed a powerfeedback signal. The voltage signal on line 16 is supplied to device 46where it is subject to modification in a manner to be describedsubsequently. The voltage signal output of device 46, I_(VMR), on line48, and the power feedback signal I_(CMR) on line 66 are supplied to acomparison circuit comprising devices 50 and 70. The comparison circuitcompares the voltage feedback signals and the power feedback signalswith a reference signal, I_(REF), applied by line 72 to device 70, toprovide, at its output line 74, a control signal, I_(CONT). The signalis appropriately modified by devices 76 and 78 to provide on line 28 anexciter field current I_(F) which is supplied to the field of theexciting generator 24. The control signal produced by the comparisoncircuit thus modifies the field excitation of the generating means tolimit its output within the predetermined maximum voltage, power, andcurrent limits which were described with reference to FIG. 2. Theabove-described aspects of the generator regulating system of thepreferred embodiment are for the most part disclosed in U.S. Pat. No.3,105,186 and in Parts 12 through 14 of "Electronics on the Rails" byRobert K. Allen published in "Railway Locomotives and Cars" about1966-1967.

Reference is again made to FIG. 1 for a further description of theregulating system. The volage feedback signal on line 16 is supplied toone input of summer 46, a voltage measuring reactor (VMR). Assubsequently described a speed control member 92, such as a foot pedal,controls the output of voltage control circuit 94, voltage controlsignal I_(VC), which is applied by line 96 to a second input of summer46. The output of device 46, a current I_(VMR), is supplied by line 48to one input of gate 50. The current feedback signal I_(G) on line 52 issupplied to one input of summer 54, a current measuring reactor (GMR).The output of summer 54, a current I_(CMR) on line 66, is supplied to asecond input of gate 50. The output of the gate 50, consisting of theinput signal having the larger amplitude, is supplied by line 68 to oneinput of summer 70.

A reference current signal, I_(REF), on line 72, is applied to anotherinput of summer 70 so as to be subtractively combined with the signal online 68. Under normal conditions of vehicle operation, the signalI_(REF) corresponds to signal I_(PM) on line 98 which is generallyrepresentative of normally available horsepower output of the primemover 10 and in the preferred embodiment has a predetermined constantmagnitude. However, as subsequently explained, the reference currentsignal is subject to modification such as when the vehicle is engaged inpenetrating and lifting earth matter. Under such conditions thereference current signal I_(REF) is subject to modification responsiveto the lifting force applied to the earth moving means, e.g. boom andshovel. The arrangement for thus modifying the reference current signalcomprises devices 100, 102 and 106.

The gate 50 and summer 70 thus constitute a comparison circuit whichselected the greater one of the output signals of the VMR summer and theGMR summer and compares the greater of these signals with the referencesignal I_(REF) to produce a control current signal I_(CONT), on line 74.In the preferred embodiment the control current signal is produced onlyif the larger one of the VMR and CMR output signals has a greateramplitude than the reference current signal. In the preferred embodimentthe comparison circuit comprising the gate 50 and summer 70 is areference mixer bridge circuit of the type disclosed in U.S. Pat. No.3,105,186.

The above-described arrangement assures that the output of thegenerating means is within predetermined maximum voltage andpredetermined maximum current limits. It assures that the output voltageof the generating means cannot, for example, exceed the voltage leveldefined by line F-G of FIG. 2, and that the output current cannot, forexample, exceed the current level defined by line H-I of FIG. 2. Forexample, if the traction vehicle operates at high speed the tractionmotors develop a substantial counter emf, thus causing a high generatoroutput voltage. If the generator output voltage approaches thepredetermined maximum limit, the voltage feedback signal will exceed thecurrent feedback signal. If the event the voltage feedback signalexceeds the reference current signal, comparison of this voltagefeedback signal with the reference current signal produces a controlcurrent signal which reduces excitation and prevent further increase ofthe generator output voltage. Similarly under high load currentconditions, as encountered during low vehicle speeds, the currentfeedback signals, exceeding the voltage feedback signals and referencecurrent signals, produces control current to reduce excitation.

In addition to the above-described arrangement for limiting voltage andcurrent output of the generating means, it is necessary to assure thatthe generator output does not exceed a desired power level, such as forexample, the power output defined by segment G-H of FIG. 2. For thispurpose the preferred embodiment utilizes function generator 62. Itsinput is the voltage feedback signal V_(G) on line 16. The output of thefunction generator, current I_(FG), is supplied by line 64 to anotherinput of summer 54, the current measuring reactor GMR. Current I_(FG)modifies the output of the GMR summer 54, i.e. the current I_(CMR) online 66, which would otherwise be solely proportional to the currentreference signal I_(G) on line 52 and thus to the armature current ofthe traction motor means. Operation of the function generator 62 is nowdescribed in connection with FIG. 2. If the generator output voltage iswithin the voltage amplitude defined, for example, by segment I-H ofFIG. 2, the function generator 62 provides no output voltage and the CMRoutput signal, I_(CMR), is unaffected by the function generator. Underthese circumstances the regulator limits output current to within themagnitude defined by segment H-I. However, as the generator outputvoltage, and the voltage feedback signal on line 16 increase, anincreasing signal is applied by line 64 to CMR summer 54 and thus isadded to the current feedback signal I_(G), such that the output signalof summer 54, i.e. current I_(CMR) on line 66 is greater than that whichwould have been produced solely by the current feedback signal. Thisincrease of the CMR summer output with increasing generator voltagecauses the maximum generator output current to decrease with increasinggenerator output voltage. Therefore the current limit of the generatingmeans output approximates segment H-G of FIG. 2 instead of beingmaintained at a constant value such as defined by segment I-H. Thesummer 54 signal I_(CMR), under such conditions, limits the output powerof the generating means and therefore constitutes a power feedbacksignal.

The output of the comparison circuit, i.e. the control signal I_(CONT)output of summation circuit 70, is applied by line 74 to anamplification system which provides an appropriate excitation signal forthe generating means 12. In the preferred embodiment illustrated in FIG.1, the control signal in line 74 is applied to one input of summer 76,whose output current I_(PWM) is applied by line 80 to function circuit78. The output of function circuit 78, the excitation control currentI_(F), is as previously described, applied by line 28 to the exciter 24so as to energize the field of generating means 12.

In the preferred embodiment the summer 76 comprises a pulse widthmodulated (PWM) amplifier, and in particular a magnetic PWM, of thegeneral type disclosed in U.S. Pat. Nos. 2,886,763 and 3,105,186. Such adevice produces a train of square wave pulses whose duty cycle isvaried, i.e. by modifying the time duration or width of the respectivepulses. It comprises a saturable transformer excited by a square waveoscillator with a tapped secondary winding connected in a full waverectifier circuit to the function circuit 78. The main windings of acontrolled saturable reactor are connected in circuit with the endterminals of the secondary windings. Control windings of the controlledsaturable reactor apply controlling signals, including the controlsignal I_(CONT), to summer 76, as illustrated by lines 74, 84, and 90 inFIG. 1. Line 74 supplies the previously described control signalI_(CONT). Line 84 supplies a rate feedback signal I_(RP), preferablyderived by coupling the output signal V_(E) of exciter 24 through ratefeedback circuit 82. This provides system stability by limitingaccelerating rates and compensating for the long time constant of thegenerator field in respect to the control system response time.

Additionally, as subsequently described in the section "Prime MoverSpeed Responsive Load Control," an electrical signal ω responsive to theshaft speed of prime mover 10 is preferably supplied to the load controlcircuit 88. In the event the prime mover is overloaded such that itsrotational speed decreases below its rated speed, load control circuit88 produces a load control signal I_(LC) which is coupled by line 90 tosummer 76 so as to reduce excitation.

The presence of control signals on lines 74, 84, or 90 varies the coresaturation of the previously described saturable reactor such that thed-c signal output of summer 76, which is applied to the function circuit78, varies inversely with the summation magnitude of the control signalsapplied to summer 76. When no control signals are applied to the controlwindings, the reactor cores are saturated such that a maximum positivesignal is applied to the input of the function circuit, permitting up tomaximum excitation of the generating means. The application of controlsignals, such as control signal I_(CONT), will reduce the flux in thecores. Thus the output signal, I_(PWM), is reduced proportionately tothe sum of the amplitudes of the applied control signals, and theexcitation of the generating means is reduced. Block 78 of FIG. 1illustrates the preferred transfer function of the function circuit 78.For purpose of explanation it is assumed that the control signalsapplied to summer 76 are zero, e.g. the reference current on line 72exceeds the larger of the feedback signals applied to the input of gate50. In this case the output of summer 76, current I_(PWM) is at aminimum amplitude and the excitation field current I_(F) is at a maximumpositive value. As the control currents, e.g. I_(CONT), applied tosummer 76 increase, the current I_(PWM) increases proportionately andthe excitation field current I_(F) falls off rapidly. With furtherincrease of the control currents, and resulting decrease of currentI_(PWM), the excitation field current decreases to zero and subsequentlyreverses in polarity until it levels off at a predetermined negativeamplitude. This reversal of excitation field current provides a fastreduction of generating means output, and provides for overcoming theresidual flux of the generating means so as to permit operation, ifdesired, at substantially zero output voltage. As previously stated theoutput of summer 76, as used in the preferred embodiment constitutes atrain of sequential pulses whose time duration is minimum when thereference current on line 72 exceeds the larger of the feedback signalsapplied to gate 50, i.e. under conditions when the output of thegenerator need not be reduced. The above-recited increase of I_(PWM) isaccomplished by increasing the time duration of the individual pulses,e.g. increasing the average value of the signal on line 80.

PRIME MOVER SPEED RESPONSIVE LOAD CONTROL

Reference was made in the preceding section "Generator RegulatingSystem" to a load control arrangement wherein overloading of the primemover produces a load control signal I_(LC) which is coupled by line 90to summer 76 so as to reduce excitation. Such an arrangement isadvantageous in traction vehicles where the prime mover must supply avariable auxiliary load, i.e. a load additional to the traction motorpropulsion system. For example, in front end loaders, the prime mover,e.g. diesel engine, also energizes the hydraulic system for moving theboom and bucket assembly. The load thus imposed on the prime movervaries considerably being maximum when the bucket penetrates the pile ofearth matter and the hydraulic system is utilized to lift the boom andbucket. Under the latter conditions the engine is subject to bogging andspeed reduction. When the prime mover is overloaded under suchconditions, the excitation of the generating means, and thus theelectrical load, is reduced in the manner described below.

The prime mover 10 normally operates at a predetermined relativelyconstant speed controlled by known types of governor systems (notillustrated). The shaft speed ω of the prime mover 10 is detected, and aload control means 88, responsive to a shaft speed signal, is arrangedto generate the load control signal I_(LC) whenever the shaft speed isabnormally low, i.e. is below a first predetermined angular velocityω_(a). As the shaft speed increases above ω_(a), the load control signalI_(LC) varies as a suitable inverse function of speed until the loadcontrol signal is reduced to zero at a second angular velocity ω_(b)which, depending on the particular application of the propulsion system,can be either lower or higher than normal. In one embodiment, forexample, the normal, loaded prime mover speed as determined by thegovernor, is approximately 2100 rpm, and the load control signal I_(LC)is zero until the prime mover looses speed to 2,050 rpm. I_(LC) thenincreases with decreasing speed to 2,000 rpm and thereafter, for lowerspeeds, remains at a predetermined substantially constant level. Theload control signal I_(LC) is applied from load control 88 by line 90 tosummer 76 so as to reduce excitation of the generator 12 when the shaftspeed is below ω_(b). The signal loop comprising prime mover 10, loadcontrol 88 and components 76, 78, 24, and 12 constitute a closed loopcircuit which if desired can maintain operation along the slope of theload control signal within the range of speeds defined by ω_(a) andω_(b). Thus when auxiliary loads are applied, the electrical load of theprime mover is modified to minimize engine bogging.

The prime mover shaft speed is preferably detected by a speed sensorproviding an analog output. For example, a magnetic speed sensor can beused to provide a pulse output whose frequency is proportional to enginespeed. The pulse signal is applied to a digital to analog converter.Arrangements of this type are well known in the art, including forexample, peak clipping circuits, such as a saturating transformer,providing input signals to a single shot trigger circuit. The outputpulses of the trigger circuit are integrated to provide an analog signalhaving an amplitude proportional to the prime mover speed.

The analog signal is supplied to a transistor amplifier, in load controlcircuit 88. The amplifier is biased to normally conduct and to produce apredetermined output current with no applied input signal. The amplifieris biased such that conduction is decreased, and output current reduced,when the analog signal is proportional to speed in excess of ω_(a).Conduction is cut off and the output current is zero when the analogsignal is proportional to a speed ω_(b). Thus load control circuit 88has a sharp cut off characteristic such that the load control signalI_(LC) is strongly increased as a result of a relatively small reductionin prime mover speed.

It should be noted that this arrangement for modifying excitation doesnot in any manner modify the maximum available traction motor voltageand current limits as established by I_(REF), I_(VMR), and I_(CMR) (asdescribed in the section entitled "Generator Regulating System"). Forexample, as a front end loader penetrates a pile it exerts maximumtorque but operates at a very low speed. The horsepower output of thegenerating means is a function of the product of speed, i.e. armaturevoltage, and torque, i.e. armature current. Therefore under suchconditions, the horsepower output of the generating means is generallybelow the maximum available horsepower. However, armature current ismaximum under such conditions. When the hydraulic system is concurrentlyactivated to move the boom and bucket, the load on the prime mover issuddenly increased. In response to the resulting reduction of primemover shaft speed, the above-described load control system reduces theload of the generating means. This is accomplished, however,independently of the parameters (I_(REF), I_(VMR), and I_(CMR)) of theregulating system which produces control signal I_(CONT). Accordingly,the maximum available current limit is not modified, and the tractionmotors can utilize maximum armature currents. Similarly at high speedoperation, the prime mover speed responsive load control system does notreduce the maximum available armature voltage.

VOLTAGE LIMIT SPEED CONTROL

Operation of certain off-highway traction vehicles, such as front endloaders, is subject to sudden and substantial modifications ofpropulsion torque. For example, front end loaders may travel underconditions requiring relatively low propulsion torque, but may suddenlyand repeatedly penetrate piles of earth matter so as to be subjected torepeated major increases of propulsion torque. These repeated rapid andsubstantial variations in propulsion torque make it desirable to providefor automatic regulation of torque and to make operator control of thetraction vehicle substantially independent of torque, while assuringthat the previously described predetermined maximum voltage, current,and horsepower output limits of the generating means 12 are notexceeded. The previously described regulating system compares feedbacksignals representative of generator voltage and current to derive apower feedback signal and compares these with reference signals togenerate control signals to limit the output of the generating meanswithin such predetermined maximum voltage, current, and horsepowerlimits. Operator control of the vehicle is attained by a moveablecontrol member connected to produce a voltage control signalrepresentative of the position of the control member. This voltagecontrol output signal is coupled in circuit with the regulating meansfor comparing the voltage feedback and reference signals so as to reducethe maximum voltage output of the generating means below thepredetermined maximum voltage limit, with minimal modification of themaximum horsepower and current limits of the generating means. In thearrangement of FIG. 1, the position of the moveable control member 92modifies the output, I_(VC), of the voltage control circuit 94. Apreferred embodiment of the latter circuit will be described below inthe section "Voltage Control Circuit." The voltage control signal I_(VC)is added by summer 46 to the voltage feedback signal V_(G), such thatsignal I_(VMR), the output of summer 46, limits the maximum generatoroutput to an output voltage, and thus to a vehicle speed, determined bythe position of the control member. Member 92 preferably is a foot pedalnormally spring biased in its upper most position. When the pedal is inthis position, the level of signal I_(VC) corresponds approximately tothe signal V_(G) which is produced without presence of an I_(VC) signalwith maximum predetermined voltage output of the generating means.Signal I_(VMR), the output signal of VMR summer 46, in such case wouldnormally exceed signal I_(CMR), the output of CMR summer 54, and wouldexceed reference current signal I_(REF), such that summer 70 of thecomparison circuit would produce a control signal I_(CONT) sufficient toreduce the maximum voltage output of the generating means to apredetermined minimum level, e.g. slightly above zero volts. As thepedal is depressed by the operator, the signal I_(VMR) is reduced andthe maximum available voltage output of the generating means increasesproportionately with the amount of pedal depression.

Operation of the voltage limit speed control can be explained withreference to FIG. 2. As indicated before, depression of the pedal 92selectively increases the maximum available voltage output, until theultimate predetermined limit F-G is reached. When the operator initiallyslightly depresses the pedal 92, the I_(VC) signal on line 96 isslightly reduced from its quiescent maximum value. This permits themaximum voltage output of the generating means 12 to increase from zeroto a first magnitude such as, for example, voltage level 1 of FIG. 2.Initially the generator output increases from zero substantiallylinearly along the line ACB, since armature current increases and thegenerator output voltage is proportional to the product of armaturecurrent and motor resistance. The traction motor torque is a function ofthe product of motor field flux and of armature current. When the motorarmature current has increased sufficiently to develop an adequatestarting torque, the vehicle begins to move, e.g. at point C of FIG. 2.At this point the vehicle develops adequate starting torque whichexceeds the subsequently required rolling torque. Therefore, subsequentto starting the available torque will exceed the load demand, causingthe vehicle to accelerate. As the vehicle accelerates the tractionmotors develop back emf, and the generator output voltage rises sincethis voltage output equals the sum of the back emf and the product ofmotor armature current and motor resistance. As shown by the dashed linecommencing from point C of FIG. 2, the generator output voltage rises tovoltage level 1. This is the maximum voltage output of the generator asestablished by the position of pedal 92. Any further acceleration of thevehicle results in increased back emf. Since the generator outputvoltage can increase no further the IR product, i.e. the armaturecurrent, must decrease. This is illustrated by the horizontal dashedline portion extending to point D of FIG. 2. The traction motor torqueis responsive to the load demand. Thus for a given operating condition,e.g. a level road and a fixed rolling resistance the vehicle willeventually maintain a constant speed and a constant armature current,e.g. as shown at point D.

If the pedal 92 is subsequently further depressed, the generator outputis permitted to rise to a higher maximum voltage level, such as forexample, voltage level 2 of FIG. 2. The increased generator voltageoutput requires an increase of the back emf and/or the product ofarmature current and motor resistance. The back emf cannot increaseuntil motor speed increases, i.e. until excess torque is developed. Thusarmature current increases initially causing the required increase oftorque, of vehicle acceleration, and of back emf. This is illustrated bythe dashed line commencing upward from point D. As previously noted theline G-H represents the ultimate horsepower limit of the system. Asillustrated in FIG. 2 by the portion of the dashed line coincident withthe hyperbolic segment of G-H, the generator voltage and currentparameters cannot exceed the power limit envelope. As described aboveacceleration subsequent to the generator output having attained themaximum voltage level, i.e. voltage level 2, results in a reduction ofarmature current until the balance point E of FIG. 2 is attained. Thedescribed arrangement is particularly advantageous for vehicles subjectto intermittent changes of load demand, such as front end loaders, sincethe current is automatically varied to match the load demand. Thisinhibits excessive acceleration of the motors, i.e. spinning, due toload changes. Operation is also maintained within ultimate current,voltage, and power limits, and the operator need only control vehiclespeed, e.g. with the speed control pedal.

CMR, VMR, REFERENCE CURRENT, AND COMPARISON CIRCUITS

FIG. 3 illustrates relevant circuit portions of the regulating system ofthe generating means 12. Current measuring reactor (CMR) 54 has controlwindings, identified as 52 and 64, for respectively applying the currentfeedback signal I_(G) and the output signal I_(FG) of function generator62. The signal I_(G) may be derived directly from current sensing means260 in the output circuit of the generating means 12, as shown in FIG.4, or derived from a summation device 60 as shown in FIG. 1. The outputsignal I_(FG) is derived from a function generator 62 which may be ofthe type disclosed in U.S. Pat. No. 3,105,186. The main windings of theCMR are connected in series circuit with a source of a-c current 120 andbridge rectifier circuit 122 by lines 66' and 66". Similarly voltagemeasuring reactor (VMR) 46 has control windings identified as 16 and 96,for respectively applying the voltage feedback signal V_(G) and thevoltage control signal I_(VC). The voltage feedback signal is derived byconnecting the voltage control winding through resistor 124 to theoutput lines 202 and 204 of the generating means 12, shown in FIG. 4.The main windings of the VMR are connected in series circuit with asource of a-c current 126 and bridge rectifier circuit 128 by lines 48'and 48". Bridges 122 and 128 are connected serially in a referencecurrent network. The reference current network comprises, in the orderstated, a source of positive potential, resistor 130, bridges 122 and128, reference current potentiometer 132, the parallel combination ofnormally closed relay switch 134 and potentiometer 136, and the commonterminal, e.g. negative source of potential. An output circuitcomprising serially connected diode 138, resistor 140, and controlwinding 74 is connected across reference bridges 122 and 128. Diodes139, 141, and 143 are connected in series across bridges 122 and 128 andare poled to limit the maximum output current.

In the above-described arrangement the impedance of the CMR 54 controlsthe flow of a-c current, i.e. from source 120 through rectifier bridge122, and the impedance of the VMR 46 controls the flow of a-c current,i.e. from source 126, through rectifier bridge 128. The effectiveimpedance of the CMR and thus the rectified a-c current through the CMRand bridge 122 is proportional to the sum of the signals applied to thecontrol windings of the CMR. Similarly the effective impedance of theVMR and thus the rectified a-c current through the VMR and bridge 128 isproportional to the sum of the signals applied to the control windingsapplied to the VMR. The reference current flows in the above-describedreference current network, and diode 138 is poled to block the passageof reference current through the output circuit. As disclosed in U.S.Pat. Nos. 2,883,608 and 3,105,186, a control signal current flowsthrough the output circuit only when either or both of the CMR and VMRoutputs has a magnitude in excess of the reference current. Theamplitude of the control current will then be a function of thedifference between the larger one of the currents I_(VMR), I_(CMR), andthe reference current, I_(REF).

PWM CIRCUIT

The output of the comparison circuit, control signal I_(CONT), isapplied to a control winding 74 of pulse width modulator, i.e. PWMsummer 76. The PWM summer comprises a saturable transformer 142 whoseprimary winding is excited by a source of a-c 144. A single square waveoscillator is utilized to provide the a-c excitation for previouslyreferenced a-c sources 120, 126, and 144, since these must besynchronized. The tapped secondary windings 145 of transformer 142 hasits end terminals connected in a series circuit comprising diode 146,the first and second main windings of controlled saturable reactor 148,and diode 150. The junction of the first and second main windings areconnected to a common terminal. This circuit constitutes a full waverectifier circuit whose output, I_(PWM), appears between line 80,connected to the center tap of transformer secondary winding 145, andthe common terminal. The inputs to the PWM summer are applied to controlwindings on the saturable reactor 148. Thus the control signal,I_(CONT), is applied to control winding 74, the rate feedback signalI_(RP) to control winding 84 and the load control signal I_(LC) tocontrol winding 90. The output of the PWM summer on line 80 comprises atrain of sequential square waves. If no control signals are applied tothe control, the cores of saturable reactor 148 are saturated and thePWM output comprises a train of rectangular pulses having maximum timeduration, i.e. the output has a maximum average value. This permitsmaximum excitation of the generating means. The presence of signals onthe windings of saturable reactor 148 will reduce the flux in thewinding cores proportionately to the sum of the applied control signals,and thus reduce the time duration of the pulses, and thus reduce theaverage amplitude of the pulse train.

GENERATOR EXCITER FIELD FUNCTION CIRCUIT

Exciter Field Function Circuit 78 receives the I_(PWM) signal on line 80and provides an appropriate excitation current I_(F) to the field 152 ofexciter 24. The regulating system operates such that maximum excitationcurrent is permitted to flow when no control signals are applied tosummer 76. As the magnitude of the control signals increases, e.g.I_(CONT) increases, the excitation current I_(F) decreases sharplytoward zero. Further increase of I_(CONT) causes a reversal ofexcitation current I_(F) until the excitation current reaches apredetermined magnitude in the reverse, or negative, direction. Thisarrangement permits a sharp reduction of excitation current and thus ofthe output of the generating means 12. The reversal of excitationcurrent upon application of a large control signal, permits fullgenerator shut off. The negative field current produced, for example,when no pressure is applied to pedal 92 overcomes any residualmagnetization in the generating means 12 and thus provides a zerovoltage output. The magnitude of negative field current is limited bypreviously described diodes 139, 141, and 143 connected in parallel withthe output of comparison circuit 70. An increase of control currentI_(CONT) causes a commensurate increase of the voltage across thecomparison circuit output 74. However, when this voltage reaches apredetermined magnitude, diodes 139, 141, and 143 conduct such that theoutput voltage does not increase with further increases of the controlsignal.

The I_(PWM) signal on line 80 is supplied through resistor 170 to PNPtransistor 172 of function circuit 78. The emitter of 172 is connectedto a common terminal, 173, and the collector is connected throughresistor 174 to a positive terminal 175. In the preferred embodimentthese terminals are supplied by a 28 volt battery. NPN transistor 160and PNP transistor 158 are connected serially across the negative andpositive terminals. Transistor 158 has its collector connected to thepositive terminal and its emitter to the collector of transistor 160 andtransistor 160 has its collector connected to common terminal 173. Thejunction between resistor 174 is connected by diode 176 to the base oftransistor 158 and by serially connected diodes 178 and 180 to the baseof transistor 160. Diodes 176, 178, and 180 are poled to passbase-emitter current of their respective transistors. Thus diode 176 ispoled in reverse in respect to diodes 178 and 180.

The exciter 24 comprises the exciter field winding connected in a bridgecircuit. Resistors 154 and 156 are connected serially from the positiveterminal 175 to the negative terminal 173. Free wheeling diodes 166 and168 are connected in parallel, respectively with resistors 154 and 156,and free wheeling diodes 162 and 164 are connected respectively acrosstransistors 158 and 160. The exciter field winding 152 is connected fromthe junction of devices 154, 156, 166 and 168 to the junction of devices158, 160, 162, and 164.

The I_(PWM) signal applied to the base of transistor 172 comprises atrain of pulse width modulated pulses, i.e. positive pulses of varyingwidth displaced by intervals when the signal is substantially zero.During time intervals when the input is positive, transistor 172 isgated on causing a reduction of its collector voltage and turn on oftransistor 160. When transistor 160 conducts, current flows frompositive terminal 175 through resistor 154, exciter field 152 andtransistor 160. This constitutes the normal, i.e. positive, direction ofcurrent flow. At the expiration of one positive pulse when the voltageat the base of transistor 172 drops to zero that transistor is cut offcausing transistor 160 to cut off and transistor 158 to turn on. Currentnow flows from the positive terminal 125 through transistor 158 exciterfield 152 and resistor 156. This constitutes the reverse, i.e. negative,direction of current flow. The described bridge circuit consequentlyalternately supplies positive and negative field current. The net fieldcurrent is dependent on the modulation of the control signals and thusthe respective time duration of positive and negative conduction.

VOLTAGE CONTROL CIRCUIT

The voltage control circuit 94 supplies a voltage control signal I_(VC),on line 96, to the voltage measuring reactor, VMR 46, representative ofthe position of operator pedal 92. When the pedal is in its uppermostposition a maximum I_(VC) signal is produced, which in turn reducestowards zero the output voltage of the generating means 12 so as toprevent movement of the vehicle. Increasing depression of the pedalreduces the I_(VC) signal and thus permits the output voltage of thegenerating means to increase so as to permit increasing vehicle speed.Pedal 92 is ganged to the arm 182 of potentiometer 184. Thepotentiometer is connected in a series circuit between a source ofpositive potential 175 and a common terminal 173, being connectedthrough resistor 186 to source 175 and through serially connected diodes188, 190, 192 to terminal 178. Potentiometer arm 182 is connected in aseries circuit, comprising diode 194, resistor 106, diode 198 to thebase of transistor 200. Resistors 202 and 204 are connected seriallyfrom the base of NPN transistor 200 to the common terminals and diode206 is connected from the base to the junction of these resistors and ispoled to assure that the emitter voltage is not positive in respect tothe base of transistor 200. The collector is connected through resistor208 to the source of positive potential.

The emitter of device 200 is connected to the base of transistor 210,whose base is coupled to the common terminal by serially connectedresistor 212, rheostat 214, line 96 and the associated control windingof voltage measuring reactor 46, and resistor 216. The collector of NPNtransistor 210 is connected through resistor 218 to positive terminal175.

When the pedal 92 is in its uppermost position, a maximum positivevoltage is applied to the base of transistor 200, so as to fully turn iton. Accordingly the signal applied to the base of transistor 210 causesthe latter to turn fully on causing maximum flow of current I_(VC)through winding 96. As the pedal is depressed, arm 182 is lowered andthe drive on transistors 200 and 210 is accordingly reduced, so as toreduce current I_(VC) on lines 96. Diodes 188, 190, and 192 provide abias to assure that full generator output is obtained upon fulldepression of pedal 92.

Arm 182 of potentiometer 184 is additionally connected through diode 194and resistor 218 to the base of transistor 220. The collector oftransistor 220 is connected through relay solenoid 222 and resistor 224to positive terminal 175 and the emitter is connected to common terminal173. Resistors 226 and 228 are serially connected between the positiveand common terminals, and relay contactor 230 is connected from thejunction of these resistors to the base of transistor 200. This circuitis intended to provide protection against breaks that might occur in thewires associated with pedal operated potentiometer 184. The pedal may belocated on the floor of the vehicle cab, and wires leading topotentiometer 184 might therefore be somewhat more subject to breakage.In the event of breakage the potential on potentiometer arm 182 or diode194 could drop to zero and a reduction of the signal I_(VC). Such acondition could result in undesirable acceleration of the vehicle.

Under normal operating conditions, potentiometer arm 182 has sufficientpositive potential to maintain transistor 220 in conduction. This isassured by the bias across diodes 188, 190, and 192. Accordinglysolenoid 222 is normally energized and contactor 230 is open. In theevent the potential drops at the base of transistor 220, for example dueto a break in the line to potentiometer arm 182, transistor 220 is cutoff. Contactor 230 then closes and supplies sufficient positivepotential to the base of transistor 200 to produce a maximum signalI_(VC) and to retard travel of the vehicle.

Under specified operating conditions such as retarding the vehicle priorto reversing its direction, it may be desirable to override the manualspeed controller and to minimize the speed of the vehicle regardless ofthe setting of the operator pedal 92. A contact 232 is provided for thispurpose. Contact 232 is connected from the junction of resistor 186 andpotentiometer 184 to the junction of resistors 196 and 218. When thiscontact is closed by an associated relay solenoid, an adequate positivepotential is supplied to the base of transistor 200 to reduce towardszero the speed of the vehicle.

ARRANGEMENT FOR MODIFYING TRACTION POWER RESPONSIVE TO LIFTING FORCE

Certain traction vehicles, such as front end loaders, have means such asa boom and bucket for penetrating and lifting matter during propulsionof the vehicle. Under such conditions it is desirable to properlyapportion the distribution between the lifting force and the tractiveeffort. For example, a front end loader penetrates a pile of earthmatter with substantial tractive effort, since torque is automaticallyincreased upon an increase of load resistance. If an excessive cut istaken, excessive power is required to lift the cut matter. Under suchconditions hydraulic lifting systems for boom and bucket assembilesoperate at very slow speeds. In addition, the extremely high liftingforce applied under such conditions displaces the loads imposed on theforward and rear axles of the vehicle. An excessive weight in the bucketof the front end loader excessively reduces the tractive effort providedby the rear wheels. Accordingly it is desirable to limit the cut toreasonable levels. This has been attained by limiting tractive effort asa function of the lifting force applied to the earth moving means, e.g.boom and bucket means. Accordingly an arrangement is provided formodifying the regulating system responsive to the magnitude of thelifting force so as to limit the output of the generating means 12 andthus the tractive effort of the vehicle. In a hydraulic lifting systemthe hydraulic pressure applied to the lifting pistons is a function ofthe lifting force, and this hydraulic pressure may be utilized to limitthe output of the generating means. In the preferred embodiment this isaccomplished by modification of the reference current. The sectionentitled "Generator Regulating System" stated, in connection with FIG.1, that the reference signal I_(REF) generally may have a constantmagnitude representative of the normally available horsepower output ofthe prime mover 10, but that this reference signal, I_(REF), was subjectto modification responsive to the lifting force applied to the boom andshovel. The arrangement for accomplishing this is illustrated in FIG. 3.A hydraulic lifting system 106 typically comprises a hydraulic reservoir234 containing fluid pumped by pump 236 which may be driven by primemover 10, and is supplied through a control valve 238 to the hoistcylinder 240. The dot-dashed lines schematically indicate the closedloop fluid path. Line 242 constitutes the fluid line segmentinterconnecting the control valve and the hoist cylinder. Line 104interconnects line 242 to hydraulic pressure switch 102. Switch 102comprises a relay solenoid 108 which is energized when the fluidpressure in the line 242 exceeds a predetermined magnitude. Uponenergization at the solenoid, normally closed switch 134 is opened.Switch 134 normally shunts potentiometer 136 in the reference currentcircuit. Thus when the fluid pressure in line 242 exceeds apredetermined magnitude and switch 134 opens, the resistance of thereference current network increases by the resistance of potentiometer136. As described in the prior section "CMR, VMR, Reference Current andComparison Circuits," the reference current circuit comprises devices122, 128, 130, 132, 134, and 136. The increased resistance of thiscircuit decreases the reference current I_(REF) applied to summer 70.Accordingly the control signal I_(CONT) output of the comparison circuitis modified so as to further limit the output of the generating means12. In the above-described embodiment while lifting force, i.e.hydraulic pressure, exceeds a predetermined magnitude transducer 102 isactuated to produce a stepped change of reference current and thus astepped reduction of the attainable output of the generating means. Forexample, in the preferred embodiment such reduction occurs when thehydraulic pressure exceeds 1800 psi. For some applications a continuousanalog rather than a stepped system could be utilized to provide gradualcontrol.

GENERATOR AND TRACTION MOTOR CIRCUIT

Reference is now made to FIG. 4. This illustrates the exciter comprisingthe exciter field 152, the exciter armature 244 and additional exciterfield winding 246. Energization of exciter field 152 was described inthe prior section "Exciter Field Function Circuit" in connection withFIG. 3. The exciter armature 244 and exciter field 246 are connected ina closed loop series circuit with field 248, of generating means 12, andswitch 26. One terminal of exciter armature 244 and of switch 26 areconnected to a common or, i.e. negative, terminal 173. The junction ofarmature field 246 and of the generator field 248 provides a source ofexciter armature voltage, V_(E), which may be coupled such as throughresistor 250 to provide the source of the rate feedback signal which isultimately applied to winding 84 of the pulse width modulator, shown inFIG. 3.

The generating means 12 comprises the generator field winding 248,armature 252 and additional generator field 254. Armature 252 and field254 are connected serially with one terminal of the armature beingconnected to positive generator output line 256 and one end of field 254being connected through current sensing means 260 to negative generatoroutput line 258. The output of the current sensor 260 provides thegenerator output current signal, I_(G), to winding 52 of the currentmeasuring reactor, CMR, as shown in FIG. 3. The traction motors areenergized by lines 256 and 258.

FIG. 4 illustrates front traction motor 2 and rear traction motor 4connected oin parallel across lines 256 and 258. Obviously more or lessmotors may be utilized as required. For example, four motorized wheels,each comprising a traction motor, may be desirable. As shown in FIG. 4,traction motor 2 comprises an armature 262, a serially connectedcommutating field 264, and a separately excited field 265. One end ofarmature 262 is connected through diode 266 to line 256 and one end offield is connected to line 258. Diode 266 is poled for conduction in thenormal propulsion mode. Resistor 268 shunts diode 266 and permitsreverse motor armature conduction through the generator armature 252during retarding operation thereby dynamically braking the vehicle.Armature 270 and commutating field 272 of motor 4 are connected in thesame manner between lines 256 and 258 by means of diode 274 and resistor276, which shunts diode 274. Motor 4 additionally comprises a separatelyexcited field 273.

Fields 265 and 273 are excited by a motor field supply such as theillustrated exciter generator 18 comprising armature 278, commutatingwinding 280, and separately excited field windings 282 and 284.Commutating winding 280, exciter armature 278 and motor field windings265 and 273 are connected in a series closed loop circuit, with thejunction of fields 280 and 273 being connected to negative output line258. The excitation applied to the separately excited field windings 282and 284 establishes the output of field supply generator 18, and thusthe traction motor field flux. As described in the section "Motor FieldExcitation," the field excitation applied to exciter generator 18normally has a predetermined magnitude. This is supplied by winding 284.When unusual tractive effort is required as indicated by traction motorcurrents in excess of a predetermined magnitude, a field boost circuitcomprising winding 282 provides additional exciter field excitation soas to increase motor torque. Under conditions when the output voltage ofthe generating means approaches a maximum value, the circuit includingwinding 284 reduces, i.e. tapers, the output of field supply generator18, and thus the field of the traction motors so as to permit operationat higher speeds.

FIELD CURRENT BOOST CIRCUIT

Under normal operation, i.e. without field boost, the separately excitedtraction motors preferably have a predetermined constant fieldexcitation. This is advantageous in applications, such as front endloaders where motor load varies substantially. For example, the loadincreases rapidly when the loader enters an earth pile, and the loaddecreases rapidly when the loader breaks out of a pile. Variations ofmotor torque cause a proportional variation of armature current. Insystems utilizing series motors, as opposed to separately excited motorshaving a constant field strength, the field varies proportionately witharmature current. This variation in turn modifies back emf and thusresults in a substantial change of motor speed. Therefore if a loaderwere driven by series motors it would experience a sudden increase ofspeed as a result of the abrupt reduction of torque upon breaking out ofa pile. Such modifications of motor speed as a function of torque areobviously undesirable and defeat the ability of the operator to controlvehicle speed by means of the previously described speed control system.While separately excited motors having a constant field flux avoid thisproblem, their maximum torque is undesirably limited in view of thepreviously described limits imposed on armature current. Accordinglyprovisions are made to augment, or boost, traction motor field flux toproduce required maximum motor torque.

Specifically the field boost circuit increases field flux responsive tohigh armature currents. The preferred embodiment provides for fieldboosting as a function of armature current above a predeterminedmagnitude which is in excess of motor armature currents encounteredduring dynamic retarding. The above-described field boost isundersirable during dynamic retarding since the field boost increasesterminal voltage and thus armature current. This positive feedbackcondition has been avoided by keeping the field boost circuit inactiveso long as armature current is lower than the maximum output currentproduced by the motor during dynamic retardation.

The above-described undesirable speed variations resulting from loadchanges are minimized by limiting field boost within parameters so as tominimize changes of back emf. The normal motor field current is of sucha high magnitude that each traction motor is magnetically saturated,whereby variations of field current produced by the field boost circuitonce activated cause only small variations in motor field strength andback emf and thus result in only minimal variations of vehicle speed.For example, if a front end loader incorporating the describedarrangement breaks out of a dirt pile, the resulting reduction of load,armature current and field current does not result in a suddenundesirable increase in back emf.

Vehicles such as front end loaders also encounter substantial loadvariations between the front and rear traction motors. For example, whenthe bucket of a front end loader is pushed down into a pile, the load onthe front wheels, and thus on the front traction motors, may be sharplyreduced in respect to the loads in the rear. Conversely when the loaderis in the hoist mode, i.e. is actually lifting matter, the load on therear wheels, and thus on the rear traction motors, may be sharplyreduced in respect to the loads in the front. In order to assureappropriate application of field boost, the field boost circuit isresponsive to the largest one of the individual motor armature currents.Responsive to an armature current above the predetermined magnitude,substantially equal field boost currents are applied to all tractionmotors. This minimizes motor slip even of motors which have no appliedload, since all traction motors are supplied with substantially the samearmature voltages and substantially equal field flux.

The field boost circuit of the preferred embodiment is now described inconnection with FIG. 4. Commutating field 264 of motor 262 is not areactive member contributing to the generation of motor back emf.Therefore the voltage across field 264 is a pure function of motor 2armature current and is utilized to provide a signal proportional toarmature current. Field 264 is connected in series circuit with diode286, exciter field winding 282 and zener diode 40'. Similarly thevoltage across commutating field 272 is a pure function of motor 4armature current. Diode 288 is connected from the junction of armature270 and commutating field 272 to the junction of exciter boost field 282and diode 286. Diode 286 is connected from the junction of motorarmature 262 and commutating field 264 to field 282, and both diodeshave their cathodes connected to field 282. The diode arrangement thusconstitutes an OR gate connected such that only that diode connected tothe commutating field with the higher voltage is subject to conduction.Such conduction occurs when this higher voltage exceeds the sum of thebreakdown voltage of the zener diode 40' and the potential drop acrossthe exciter field 282 and diode 286. Thus if the voltage at the junctionof armature 262 and commutating coil 264 is of sufficient magnitude, andexceeds the voltage at the junction of armature 270 and commutatingfield 272, diode 286 conducts, and current I_(B) through the exciterboost field 282 is then a function of current through armature 262.Accordingly, when armature current exceeds a predetermined threshold thecurrent through exciter field 282 generates field flux which boosts theexciter field flux produced by exciter field 284, thereby causing theexciter 18 to supply to the field windings 265 and 273 of the respectivemotors 2 and 4 corresponding increments of field excitation which willequally increase the torques of both motors. As armature currentincreases above the breakdown threshhold there is a commensurateincrease of current through field 282 and additional boost.

Obviously means other than zener diode 40' may be utilized to controlthe activation of the boost field excitation means. The preferredembodiment utilizes a maximum current limit of 1200 amperes per motor,and the parameters of the field current boost circuit were sized toprovide for field boost commencing with motor currents in excess of 800amperes. Free wheeling diodes 292 and 294 are connected, respectively,across fields 282 and 284 to protect the fields and in the case of diode292 also zener diode 40' and diodes 286 and 288. Fields 282 and 284 areconnected in circuit through respective reversing contactors 34' whichmay be ganged and operated by a single reversing means in order toreverse motor direction.

VOLTAGE FUNCTION FIELD TAPER

As described in the preceding sections "Motor Field Excitation" and"Generator and Traction Motor Circuit," the traction motors normallyoperate with substantially constant and equal field excitation,established by the field excitation of winding 284 of exciter generator18, illustrated in FIG. 4. The exciter field winding 284 is connected ina series circuit comprising positive potential source 300, resistor 304,exciter field winding 284, the parallel combination of transistor 308and resistor 306, and negative bus 302. During normal operation,transistor 308 is fully conducting, such that a substantially constantexcitation current I_(E) flows through winding 284. This current isdetermined by the magnitude of resistor 304, and it is selected so thatthe resulting motor field current produced by the exciter generator 18is adequate to magnetically saturate the motors 2 and 4 as previouslyexplained.

During normal operation the vehicle speed is controlled by the voltagelimit speed control. If it is desired to increase the speed, the maximumavailable generator voltage, and thus the traction motor armaturevoltage, is increased up to the ultimate predetermined maximum voltageoutput of the generator. In order to attain even higher speeds, thefield is weakened as the ultimate maximum generator voltage isapproached. This is accomplished by tapering back, i.e. reducing, themotor field when the motor armature voltage exceeds a predeterminedmagnitude. Such taper does not commence until the armature voltageclosely approaches the ultimate maximum armature voltage such that thefield is maintained constant during normal operation in order to assureaccurate horsepower and speed control.

In the preferred embodiment field taper control is effected by graduallyand smoothly reducing conduction of transistor 308 until transistor 308is fully cut off. Thus the current flow through field 284 is reducedfrom a normal value I_(E) by an increasing taper, i.e. I_(T). Whentransistor 308 is fully cut off, the field current I_(E) -I_(T) reachesa minimal value determined by the sum of the magnitudes of resistor 304and resistor 306. For example, in the preferred embodiment the maximumarmature voltage limit is 700 volts. Taper, for example, commences at650 volts and continues to 680 volts, when transistor 308 is cut off.The minimal field current, existing for example from 680 to 700 volts,establishes a maximum top speed and prevents undesirable higher run awayspeeds. The taper, e.g. between 650 and 680 volts, is intended to assuresmooth transition.

The voltage function field taper circuit additionally is operativeduring dynamic retarding to regulate motor terminal voltage. Duringdynamic retarding the traction motor load comprises a resistance, e.g.268, and the armature of generating means 12. If the motor armaturevoltage increases excessively, the voltage function field taper circuitreduces, i.e. tapers, the motor field. This reduces excitation of themotor and thus prevents excessively high motor armature voltages.

During the propulsion mode the armature voltage of parallel connectedtraction motors 2 and 4 corresponds to the output voltage of generatormeans 12, and field taper can be controlled as a function of thatvoltage. However, during dynamic retardation, also termed dynamicbraking, the motor armature voltage varies independently of thegenerator output. During dynamic retardation there may be variationsbetween the armature voltages of the respective traction motors.Accordingly, the preferred embodiment senses armature voltage of theindividual traction motors and equally reduces the level of fieldexcitation of each motor by a magnitude proportional to the amaturevoltage of largest magnitude when that voltage exceeds the predeterminedreference level.

As illustrated in FIG. 4, this is accomplished by a circuit comprisingdiodes 314 and 316, rheostat 312 and control winding 318 of saturablereactor 320. Diode 314 has its anode connected to the junction ofarmature 262 and diode 266, diode 316 has its anode connected to thejunction of armature 270 and diode 276. The cathodes of diodes 314 and316 are connected to each other and to rheostat 312. The brush ofrheostat 312 is connected through the control winding 318 to bus 258.The diodes are thus connected in an OR circuit, of the type described inthe preceding section, so that the current through control winding 318is a function of the largest one of the armature voltages of motors 2and 4. During motoring both of these voltages will be virtually the sameas the output voltage of the generator 12.

Saturable reactor 320 has its main windings connected in series circuitwith bride rectifier 324 and a source of square wave 324, such that theoutput of rectifier 324, as applied across potentiometer 326, isrepresentative of the armature voltage signal. The saturable reactorcircuit arrangement is similar to the VMR and CMR circuits described inthe section "CMR, VMR, Reference Current and Comparison Circuits," and acommon source of square waves is utilized.

Operational amplifier 328 has one input connected to the arm ofpotentiometer 326 and a second input to the arm of potentiometer 330.One end of potentiometer 330 is connected to negative bus 302 and itsother end is connected through potentiometer 322 to a source of positivepotential. A zener diode 334 is connected across potentiometer 330, soas to maintain a predetermined reference potential across thepotentiometer. Under normal operating conditions, amplifier 328conducts. As the armature voltage exceeds the predetermined threshhold,at which tapering is to commence, the potential applied frompotentiometer 326 to the one input of amplifier 328 increasessufficiently to reduce its conduction. This threshhold can be adjustedby means of potentiometer 330. The output of amplifier 328 is connectedto the input of inverting amplifier 336. The latter is normally cut off,and commences to turn on as amplifier 328 commences to turn off. Theoutput of amplifier 336 is connected to the base of NPN transistor 338,whose collector is connected to a source of positive potential and whosecollector is connected to the base of PNP transistor 308. Base resistors340 and 342 are connected from the base, respectively, of transistors338 and 308 to negative bus 302. As amplifier 336 turns on, normally cutoff device 308 cuts off. Thus turn off of device 308 occurs as afunction of armature voltage, and the threshhold where field tapercommences is adjustable.

The described field taper arrangement is adjusted so as to preclude anyreduction in field strength until the armature voltage approaches theultimate voltage limit, thereby permitting desired speed and torquecontrol operation over the normal operating range and providing forgradual taper and thus smooth variation of speed. Additionally, taperoperates as a direct function of armature voltage. Since the field istapered as a direct function of high armature voltage, it is effectiveon an "as needed" basis during the propulsion mode and is available toregulate motor terminal voltage during the retarding mode.

While a preferred form of the invention has been herein shown anddescribed by way of illustration, modifications and variations thereofwill probably occur to persons skilled in the art. It is thereforeintended by the concluding claims to cover all such changes andmodifications as fall within the true spirit and scope of thisinvention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. For use in an electrically propelled earthmovingtraction vehicle of the type having integral earthmoving means forpenetrating, lifting, transporting, and discharging earth matter andwherein prime mover means drives electric generating means supplyingelectrical energy to traction motor means, said prime mover means alsobeing adapted to supply power to means for operating said earthmovingmeans, a propulsion control system for limiting the electrical output ofsaid generating means responsive to the available output of said primemover, said propulsion control system comprising:a. a source of feedbacksignals varying as functions of the electrical output of said generatingmeans; b. a source of reference signals representative of the availablepower output level of said prime mover; c. comparison means responsiveto said feedback and reference signals and having an output adapted toprovide a control signal for limiting the output of said generatingmeans; d. means for deriving a lifting force signal representative of alifting force applied to said earthmoving means; and e. means responsiveto said lifting force signal for modifying said control signal to reducethe output limit of said generating means and thereby to reduce maximumavailable tractive effort.
 2. The arrangement of claim 1 wherein saidcontrol signal modifying means comprises means for algebraically addingsaid lifting force signal to said reference signals to derive a modifiedreference signal which is applied to said comparison means.
 3. Thearrangement of claim 1 wherein said earthmoving traction vehicle is of atype having hydraulically operated earthmoving means for penetrating andlifting earth matter and said means for deriving a lifting force signalcomprises means for detecting hydraulic pressure applied to saidearthmoving means.
 4. The arrangement of claim 3 wherein said means forderiving a lifting force signal further comprises electric switchingmeans having a first normal state so long as the detected hydraulicpressure does not exceed a predetermined amount and having a secondstate when hydraulic pressure is above said predetermined amount, andwheren said control signal modidying means comprises means for couplingsaid electric switching means in circuit with said source of referencesignals to reduce the reference signal by a predetermined value whensaid switching means is in its second state, thereby correspondingreducing the output limit of said generating means.
 5. The arrangementof claim 1 wherein the electrically propelled earthmoving tractionvehicle is a front end loader.
 6. The arrangement of claim 5 whereinhydraulic means are utilized for lifting the bucket of the front endloader, and said means for deriving a lifting force signal comprisesmeans for detecting hydraulic pressure exerted to lift said bucket.